An engine comprising injectors, an air flow meter, an air-fuel ratio sensor and an electronic control unit is described in the Patent Document 1. The injectors are positioned on a body of the engine and inject fuel to combustion chambers when the injectors receive commands to inject fuel from the electronic control unit. The air flow meter is positioned on an air intake passage of the engine and output an output value corresponding to an amount of the air passing the air flow meter. The electronic control unit calculates the amount of the air passing the air flow meter on the basis of this output value. It should be noted that the air passing the air flow meter flows into the combustion chambers. Therefore, it may be said that the air flow meter detects the amount of the air flowing into the combustion chambers (hereinafter, this amount will be referred to as—intake air amount—). The air-fuel ratio sensor is positioned on an exhaust passage of the engine and outputs an output value corresponding to a concentration of oxygen in an exhaust gas reaching the air-fuel ratio sensor. The electronic control unit calculates an air-fuel ratio of the mixture gas formed in the combustion chambers on the basis of this output value. Therefore, it may be said that the air-fuel ratio sensor detects the air-fuel ratio of the mixture gas formed in the combustion chambers (hereinafter, this air-fuel ratio will be simply referred to as—air-fuel ratio of the mixture gas—)
A difference may occur between an actual fuel injection amount (i.e. an amount of the fuel actually injected from the injectors) and the commanded fuel injection amount (i.e. the amount of the fuel to be injected commanded to the injectors by the electronic control unit). Further, a difference may be occur between an actual intake air amount (i.e. an amount of the air actually sucked into the combustion chambers) and a detected intake air amount (i.e. an amount of the air detected by the air flow meter). When the difference occurs between the actual fuel injection amount and the commanded fuel injection amount and the electronic control unit performs a control assuming that no difference occurs between the actual fuel injection amount and the commanded fuel injection amount, no desired performance of the engine may be obtained. Further, when the difference occurs between the actual intake air amount and the detected intake air amount and the electronic control unit performs a control assuming that no difference occurs between the actual intake air amount and the detected intake air amount, no desired performance of the engine may be obtained.
In the engine described in the Patent Document 1, an air-fuel ratio of the mixture gas is calculated using the commanded fuel injection amount and the detected intake air amount (hereinafter, this calculated air-fuel ratio will be referred to as—estimated air-fuel ratio—) and then the difference of the estimated air-fuel ratio relative to the detected air-fuel ratio (i.e. the air-fuel ratio detected by the air-fuel ratio sensor) is calculated. Then, this difference is divided into the air-fuel ratio difference which may occur due to the fuel injection amount difference (i.e. the difference between the actual and commanded fuel injection amounts) and the air-fuel ratio difference which may occur due to the intake air amount difference (i.e. the difference between the actual and detected intake air amounts). Then, the commanded fuel injection amount is amended for compensating the above-explained divided air-fuel ratio difference which may occur due to the fuel injection amount difference and the detected intake air amount is amended for compensating the above-explained divided air-fuel ratio difference which may occur due to the intake air amount difference.